New-Tech Europe Magazine | March 2018

EMC Filter for DC/DC switching controller optimized

Stefan Klein, WURTH ELEKTRONIK

Efficiency versus EMC Modern power supplies need to reduce power loss to maintain a high degree of efficiency. Switch mode power supplies and DC/DC switching controllers are state of the art and allow great efficiency, but if the design of the circuit and circuit board layout is less than ideal this can result in increased emission of radio interference voltage. This article discusses the systematic implementation of input filters for reducing symmetric interfering voltage in DC/DC switching controllers. Demand for an input filter Whether you have a switch mode

power supply or DC/DC switching controller, every type of switch mode power supply creates a broadband interference emission in the form of interfering voltage and interference fields that can make other electrical devices malfunction. The main cause for the interfering voltage is current at the input that flows through the input capacitor of the switching controller with the clock frequency of the switching controller. It causes a voltage drop VRippel via the ESR that consists of a proportional percentage by the ESR and an integrated percentage by the capacity. Measurement of the interfering voltage This can be corrected by an input

filter that reduces the amplitude of the interfering voltage, suppresses harmonics and plays a vital role in reducing the radio interference voltage to an acceptable level. For example, the generic standard EN61000-6-4 establishes at 150 kHz a limit value of the virtual peak value of 79 dBμV. The current market for passive components offers a broad portfolio of "ready- made filters" that are marketed with high insertion loss. Insertion losses between 70 dB and 100 dB are declared, for example. But these values are rarely achieved in practice because such filters were measured in a 50 Ω system and the impedances of the power supplies deviate from these values. The development of an individual filter

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